1. Field of the Invention
The present invention concerns a process and catalyst system for reduction of nitrogen oxides from exhaust gases using a reducing agent such as ammonia and urea. In particular, the invention relates to a dual bed catalyst system for reduction of nitrogen oxides using a reducing agent in which the first catalyst bed is an iron-beta-zeolite (Fe-beta-zeolite) and the second catalyst bed downstream is silver supported on alumina (Ag/Al2O3). More particularly, the present invention relates to a process and catalyst system for converting NOx in exhaust gas from lean-burn combustion engines to nitrogen by adding a mixture of hydrogen and ammonia to the exhaust gas and subsequently passing the gas over a suitable dual bed catalyst, in which the first catalyst bed is an iron-beta-zeolite (Fe-beta-zeolite) and the second catalyst bed downstream is silver supported on alumina (Ag/Al2O3).
2. Description of the Related Art
The emission of nitrogen oxides by exhaust gases in stationary and automotive applications has long been a major environmental issue. The harmful effects of nitrogen oxides (NOx) are well known, and therefore intensive research is being conducted to find processes and catalyst systems which are able to cope with stricter environmental regulations. Conventional catalysts for NOx reduction comprise vanadium; however, these catalysts are becoming less and less attractive as tightening environmental regulations are expected to forbid their use. In the automotive business, particularly in exhaust gases from lean-burn engines, the reduction of NOx to nitrogen (N2) is usually conducted by using ammonia or urea as reducing agents over a suitable catalyst in the so-called selective catalytic reduction (SCR).
Systems utilising selective catalytic reduction (SCR) of NOx by ammonia (an aqueous solution of urea can also be used as ammonia source) to remove NOx from exhaust in lean burn combustion processes is well established both for stationary and automotive applications.
In some applications, especially automotive applications, when using commercial SCR catalysts like V/W/TiO2 and Fe-zeolites, the standard SCR reaction (4NO+4NH3+O2=4N2+6H2O) is not fast enough at low temperatures (around 200° C.) to fulfil the NOx conversion requirements given by legislation in some countries. One way to obtain higher NOx conversion at these low temperatures is to take advantage of the so-called fast SCR reaction (NO+NO2+2NH3=2N2+3H2O). At normal conditions, the major part of the NOx in lean combustion exhaust is NO. Therefore, to obtain a NO:NO2 ratio close to 1:1 required for the fast SCR reaction an oxidation catalyst for oxidation of NO to NO2 is usually applied upstream the SCR catalyst. This solution has some drawbacks: 1) The oxidation catalyst required for the NO oxidation requires a high loading of precious Pt; 2) The oxidation catalyst deactivates significantly over time resulting in a change in SCR activity which makes regulation of the NH3/urea dosage difficult; 3) It is not possible to obtain the optimum NO:NO2 1:1 ratio in the whole operational temperature interval.
High SCR activity can be achieved over Cu-zeolite materials without taking advantage of the fast SCR reaction; however, Cu-zeolites are more prone to hydrothermal deactivation than Fe-zeolites, which limits their use in many applications.
U.S. Pat. No. 6,689,709 discloses the use of iron-beta-zeolites for the selective reduction of nitrogen oxides with ammonia at high temperatures (425, 550° C.). By pre-steaming the catalysts at 600 to 800° C. for 0.25 to 8 h, the catalysts are shown to be hydrothermally stable.
Richter et al. (Catalysis Letters Vol. 94, Nos. 1-2, page 115, April 2004) shows that some catalysts based on Ag/Al2O3 function well as SCR catalyst when a mixture of H2 and NH3 is used as reducing agent. In a gas with a 1:10:1 molar ratio of NH3:H2:NO and surplus of oxygen (6 vol % O2), almost full NO conversion at a temperature as low as 200° C. is achieved. However, if hydrogen is removed from the gas the NO conversion becomes more limited at all temperatures in the range 150 to 450° C. In a gas with a 1:2.5:1 molar ratio of NH3:H2:NO, i.e. with reduced amount of hydrogen and surplus of oxygen (6 vol % O2), over 90% NO conversion at 300° C. are achieved. NOx conversions close to 80% are obtained at 300° C. in a gas with 1:1.5:1 molar ratio of NH3:H2:NO. In other words, reduction of 1 mole of NO requires 1.5 to 2.5 or more moles of hydrogen. Using such a catalyst alone would require a significant amount of hydrogen to be used to obtain an acceptable NOx conversion over a broader range of temperatures, i.e. 150 to 550° C.
Our own studies on the performance of Ag/Al2O3 catalyst in H2-assisted SCR removal with ammonia (or urea) show that this catalyst in the presence of reasonable amount of hydrogen (1000 ppm) provides a very promising NOx conversion in the course of NH3-DeNOx of a gas with approximately 1:3:1 molar ratio of NH3:H2:NO within the low temperature range 175 to 250° C. However, in the absence of hydrogen, which is desired in order to keep costs down, the catalyst is not active in SCR removal with ammonia or urea Our studies on this catalyst also show that the reduction of 1 mole of NO requires a considerable amount of hydrogen, namely 1.5 to 2 moles of hydrogen. Moreover, the catalyst deactivates after repetitive catalytic cycles due to the presence of SO2 in the feed gas, particularly when exposed to high SO2 content in the gas for short periods (e.g. 30 ppm for 2 h) compared to low SO2 content in the gas for longer period (e.g. 8 ppm for 8 h).
It is therefore desirable to provide a process and a catalyst for NOx reduction which overcome the above problems.